Hydrofluorocarbons (HFCs) and hydrochlorofluorocarbons (HCFCs) are considered to be useful alternatives to traditional chlorofluorocarbons (CFCs) in a variety of application from refrigeration, and cleaning, to blow molding of polymers. These compounds have shorter atmospheric residence times and decompose at lower altitudes than CFCs, because of the presence of reactive carbon-hydrogen bonds. This behavior is less deleterious to the upper atmosphere since much less ozone is decomposed by chlorine radicals for HCFCs and since there is no ozone loss for HFCs.
Although these compounds are beneficial to the environment they are more difficult to synthesize and to purify than the CFCs. This difficulty arises from the propensity of HFCs and HCFCs to hydrogen bond with other organics and acid gases, especially HF, a common by-product of HFC and HCFC synthesis. These hydrogen bonding effects result in azeotropes and pinch-points in the vapor liquid equilibria (VLE) of mixtures of these compounds.
Pinch-points and azeotropes complicate standard approaches to separations of product streams. Pinch-points require larger columns with higher numbers of trays (plates) and azeotropes cannot be separated by standard distillation. Distillation must be done by extraction in two column systems, or the azeotrope must be "cracked" at low temperatures using cryogenic methods. In every case the cost of separation increases considerably with the complexity or size of the unit operation or operations to be employed.
The theoretical concepts dealing with the mechanisms of absorptive separations are well known to those trained in the art. Good discussions are provided by R. T. Yang, Gas Separation by Adsorption Processes, Butterworths, Boston (1987) and D. M. Ruthven, Principles of Adsorption and Adsorption Processes, Wiley, New York (1984).
In comparing the relative merits of a standard distillative separation versus a less standard absorptive separation certain criteria need to be met in order for the latter to be more efficient than the former. In most cases distillation is the most cost effective and simplest operation employed for separation. However, criteria can be established and met under some circumstances that suggest the preferential use of absorption technology. These criteria have been reviewed in a number of places and are familiar to anyone trained in the art. G. E. Keller, R. A. Anderson and C. M. Yon, "Adsorption", in Handbook of Separation Process Technology, R. W. Rosseau, Ed. Wiley, New York, (1987), pp. 644-696 have done a particular service to the field by clearly enumerating these criteria, as has G. E. Keller III, "Gas-Adsorption Processes", in Industrial Gas Separations, ACS Sym. Series 223, T. E. Whyte, Jr., C. M. Yon, E. H. Wagner, Eds., ACS, Washington (1983), pp. 145-171 in an earlier work. In particular these authors state that adsorptive separations should be considered as an alternative to distillation, when the relative volatility between the key components to be separated is between 1.2 to 1.5 or less--as is the case for azeotropic mixtures--and when separations by distillation require multiple columns or cryogenic operation. Of course these criteria require that a suitable sorbent can be identified which carries out the separation efficiently and economically.
The choice of a suitable sorbent is also made on the basis of the separation mechanism it imparts on the mixture. This is important for the following reasons. If the product stream to be separated is high in concentration of less valuable, and less strongly adsorbed component, then a simple equilibrium driven separation is ideal. The minor component builds in concentration on the sorbent surface until full capacity is reached, the feed is swung to a second column and finally it is regenerated to strip off the pure, high value component. Here the sorbent bed can be relatively small with low capital expenditures, and the cycle time can be long with low operating and process costs. Activated carbon, silica or some other sorbent can be suitable for such a simple process approach.
Similarly, if a stream rich in a value-added component is contaminated by one or more minor components, then a straightforward equilibrium-driven separation over a standard sorbent can be used, provided the impurities are more strongly sorbed than the major component.
However, this may not always hold. Specifically, if one has a stream which is contaminated by impurities, and the mixture is difficult to separate by distillation--according to the criteria mentioned--and if the impurities are not more strong sorbed than the major component, then a simple equilibrium separation may not be feasible. The reason is that in order to carry out the separation, the carbon bed would have to be too large to be economical. Process costs would be expected to scale with the bed size.
In this type of situation, which can often be the case with HFC and HCFC product streams, a different sorptive mechanism over a non-standard sorbent can be considered. Once again the key, as pointed out generally by Keller et al. (1987), is the choice of a suitable sorbent. It is often true that the more strongly "held" molecule is also larger, if all other factors are roughly equal. When this is true then "shape-selective" separations can be used. Here the separation is driven by the differential rates of uptake of components in the mixture on a molecular sieving sorbent. At one extreme the pore structure of the sorbent completely restricts access of one or more components of the mixture to the inner sorbent surface. At the other extreme the separation is based on different fluxes into the sorbent, which in turn are based on different diffusivities for different sized components.
Zeolites can be expected to show shape selective separations because of their molecular-sized pores. A variety of examples are provided in the literature (Yang, 1987), Ruthven (1984). Although useful for some separations of HFC and HCFC containing mixtures, they are not useful for the separations of mixtures that contain acid gases, especially HF since they are detrimentally reacted with and consumed by the acid.
An alternative to zeolites are the carbon molecular sieves (CMS) which are shape and size selective, but are not consumed by acid gases. Commercial applications of carbon molecular sieves are typified by the recovery of pure nitrogen from air in a pressure swing adsorption process. Although oxygen and nitrogen differ in size by only 0.2 .ANG., the separation is efficient. This arises from the fact that the rate of transport of oxygen into the carbon sieve pore structure is markedly higher than that of nitrogen. Hence, the kinetic separation works, even though the equilibrium loading levels of O.sub.2 and N.sub.2 are virtually identical, and therefore would not provide any separation. These effects have been considered by Yang (1987).
Carbon molecular sieve-based separations of fluorocarbons have been investigated for particular separations. S. F. Yates, U.S. Pat. No. 4,940,824 reports that carbon molecular sieve can be used for the removal of vinylidene chloride from HCFC-141b. In a separate disclosure, U.S. Pat. No. 4,940,825, Yates reports that dichloroacetylene is separated or removed from HCFC-141b and/or vinylidene chloride over a carbon molecular sieve with a mean pore size of 4.2-4.5 .ANG.. In both cases the examples indicate that the carbon molecular sieve strongly adsorbed the impurity molecules, thereby stripping them from the feed. In none of the examples was a CMS material regenerated or shown to be regenerable. Similarly, in S. F. Yates, U.S. Pat. No. 4,906,796 (1990) teaches that R-1122 (2-chloro-1,1-difluoroethylene) can be substantially removed from HFC-134a (C.sub.2 F.sub.4 H.sub.2) by selective sorption of the R-1122 into either CMS or 5A zeolite, with the latter preferred. Here also the key to the separation is the strong absorption of the impurity molecules into the pore structures of either the CMS or the zeolite.
For each of the cases mentioned it was noted that the carbon molecular sieves utilized were prepared from polymeric precursors that did not contain oxygen. This goes back to the teaching of Chang in U.S. Pat. No. 4,820,681. This patent describes the methodology for the synthesis of the CMS material used in each of the subsequent process patents. Particularly important to these applications is the use of a cross-linked polymer precursor which is oxygen free. An example is a polymer made from vinylidene fluoride (PVDF) and crosslinked with divinyl benzene.